1. Field of the Invention
The invention relates to structures and methods for immunizing MOS integrated circuits to ionization, such as that caused by alpha particles.
2. Prior Art
High levels of radiation, including alpha particles, can be very damaging to semiconductors and integrated circuits. For example, radiation can damage layers in integrated circuits, permanently shifting threshold voltages and thereby destroying the effectiveness of the circuit. A great deal of research has been performed on "hardening" electrical circuit so that they will tolerate high levels of radiation. For the most part, these efforts have been directed towards preventing damage from high density and high energy radiation which is incident on a circuit; that is, radiation from an external source, not radiation from within the circuit itself.
There is a certain amount of radiation, including alpha particles, present in the atmosphere and in many common materials used in the fabrication of integrated circuits and their packages. In the past, this very low density radiation has not presented any problems for integrated circuits. However, as will be described in greater detail in conjunction with FIG. 1, even a single alpha particle can present a problem for a high density (small geometry) MOS integrated circuit.